Chiral liquid crystal displays and display materials consisting generally of cholesteric liquid crystal entrapped in plastic sheets have been proposed in the past. U.S. Pat. No. 3,585,381, issued Jun. 15, 1971, discloses a color change display in which cholesteric liquid crystal is encapsulated in a suitable film. The color contrast and purity are stated to be improved by a top layer on the film which is indexed matched and conforms to the encapsulated layer. An associated U.S. Pat. No. 3,600,064 discloses various encapsulation procedures for incorporating cholesteric liquid crystal in a polymeric matrix. These patents suggest the possibility of adjusting color using an electrical stimulus or field; however, the disclosed cholesteryl esters have a small dielectric anisotropy and are relatively unresponsive to applied fields. Further, the patents do not disclose how the color is affected, and it may only be a wavelength shift of the reflected light or a phase change of the liquid crystal into a reflecting phase.
U.S. Pat. No. 3,620,889, issued Nov.16, 1971, discloses another type of system in which cholesteric liquid crystal is entrapped in plastic. When heated or cooled, the system is either clear or opaque, and a color change phenomenon occurs. U.S. Pat. No. 4,101,207, issued Jul. 18, 1978, discloses still another system in which cholesteric liquid crystal is incorporated into a polymer. As disclosed in the example of this patent, the film is placed between transparent electrodes and displays a dynamic scattering effect when subjected to a voltage.
French patent application No. 74.30349 (Publication No. 2,343,234) discloses various cholesteric liquid crystal materials dispersed in water insoluble polymers. The materials are primarily temperature sensitive, and the esters disclosed do not appear to be electrically sensitive due to their small dielectric anisotropy.
Recent developments concerning entrapment of nematic liquid crystal microdroplets in light-transmissive, polymeric matrixes are disclosed in U.S. Pat. Nos. 4,671,618; 4,673,255; 4,685,771 and 4,688,900, the disclosures of which are incorporated by reference. The developments disclosed in these patents are light-modulating materials prepared as phase-separated dispersions of liquid crystal microdroplets. The phase separation of the liquid crystal in the polymer can be carried out by polymerization, thermal induction or solvent evaporation. Such materials are referred to as polymer dispersed liquid crystal material (PDLC).
The PDLC materials are usually made with nematic liquid crystal possessing positive dielectric anisotropy. The ordinary index of refraction typically is matched to the refractive index of the polymer so that in a field-ON state the display or material appears visible or clear because the optical axes of the microdroplets are aligned parallel to the field and normal to the viewing surface. In a field-OFF state, the optical axes of the microdroplets are randomly oriented or configured so that incident light is scattered and the material or display appears opaque.